Helicobacter pylori has been shown to cause chronic active gastritis and peptic ulcer disease and is associated with the development of gastric adenocarcinoma, the fourth most common cancer in the world. Additionally, there is a suggestion that in the developing world, infection with H. pylori in childhood increases the likelihood of developing chronic diarrhea, malnutrition and growth failure. It is estimated that the yearly world medical cost to treat H. pylori gastritis and peptic ulcer alone exceeds $7.4 billion. In parts of the developing world, up to 90% of the population is infected with H. pylori by ten years of age. The exact mode of transmission and the risk factors associated with the acquisition of an H. pylori infection are largely unknown, especially in the developing world. The purpose of this study will be to describe the epidemiology of H. pylori infection in a population-based cohort of rural Egyptian children followed from birth until three years of age. Our database from this cohort will include baseline socio-demographic features, periodic monitoring of diet and anthropometric status, systematic monitoring for ascertainment of etiologically defined diarrheal illnesses, and systematic monitoring for ascertainment of prevalent and incident H. pylori infections. With these data, we will calculate age-specific incidence rates for H. pylori infection. We will also relate the occurrence of prevalent and incident H. pylori to antecedent socio- demographic, hygienic, behavioral, and dietary characteristics, to the Lewis b and ABO blood group status of the infant, and to levels of IgA anti-H. pylori antibodies and the presence of Lewis b antigen in breast milk ingested by breast-feeding infants. Finally, we will assess the association between H. pylori infection and rates of pathogen-specific diarrhea, rates of persistent diarrhea, and the risks of wasting and stunting. Because NICHD and NAMRU-3 have already established a cohort study of pediatric diarrhea in rural Egypt, the present study will represent an "add-on" to the substantial research structure already in place in our field site. In this field site, a pilot study has indicated that 87.5% of 227 mothers had serological evidence for H. pylori infection. By 18 months of age, 37/148 (25%) of babies born to H. pylori infected mothers were infected with the bacteria as compared to 3/22 (13.6%) of similar aged babies born to seronegative mothers. Hence we believe our field site is likely to provide a good setting in which to study the epidemiology of H. pylori infections.